As is known in connection with the handling of radioactive materials, the storage and transport of nuclear reactor fuel elements and/or other nuclear reactor fuels and wastes, require canisters, containers, receptacles and vessels which must be sufficiently strong so as to be capable of withstanding the normal handling and storage stresses and mechanical operation, and even unusual stresses to afford a high measure of security to personnel concerned with such receptacles. Apart from the considerable mechanical stability which must be possessed by such a vessel, it must have high absorption capability so that it shields the environment and operating personnel from gamma radiation as well as radiation of other types.
One way of providing for relatively high mechanical stability and high radiation-absorbent capability is to constitute the vessel of a laminated structure having inner and outer layers and an intermediate layer between the inner and outer layers.
In one prior-art design, the inner and outer shells are formed of welded steel construction while the intermediate layer is a cast lead layer having low mechanical strength but a high absorption capability. The lead can be cast in situ between the inner and outer welded steel shells.
While containers of the aforedescribed type have been used to receive storage and transport radioactive materials, problems are encountered when such systems are employed for the storage and transport of irradiated nuclear-reactor fuel elements. Nuclear-reactor fuel elements generate considerable heat as a result of the radioactive decomposition and/or fission processes within the radioactive material.
Thus a storage vessel or receptacle for such fuel elements can consist of a pot-shaped or cup-shaped body which is open upwardly and has walls and a base defining a chamber for the radioactive material as well as a cover for the open mouth at the upper end of this chamber.
To eliminate problems resulting from the generation of heat, the walls of the hollow body of the vessel can be provided with passages permitting the flow of a coolant in a closed cycle in heat-exchanging relationship with the walls to carry away the heat developed by the radioactive material.
In the prior-art multi-layer structure, the coolant passages are formed by tubes or pipes which are welded to the steel outer layer and/or the steel inner layer.
Various testing procedures have been set up to permit the testing of vessels for the purposes described so as to insure that they will be capable of effective use for the storage and transport of radioactive materials with a minimum danger to handling personnel and, more generally, to the environment.
For example, one such test regimen requires that the vessel be dropped in free fall through a height of 9 meters on a non-yielding surface. In another regimen, the vessel is dropped through a height of 1.2 meters on a mandrel of defined configuration. Tests are carried out at temperature of about 800.degree. C. over periods of thirty minutes.
So that the conventional fuels may withstand these tests successfully, it has been found to be necessary to make the steel outer layer especially thick to prevent complete deformation following free fall. Even with very thick welded construction of the vessel, however, it is found that the ducts or tubes welded to the steel of the vessel tend to rupture and can give rise to contamination of the environment by the release of a radioactive coolant.
Furthermore, at the temperatures at which the containers are tested and those at which the containers are used, there is a tendency for the lead intermediate layer to be melted and lose its absorption effectiveness. As a result, special insulating precautions must be taken, e.g. a moist plaster layer introduced between the steel outer layer and the lead intermediate layer.
Upon failure of the coolant cycle, moreover, there is also the danger that the lead intermediate layer may be melted by the heat generated by the radioactivity.
Finally, in connection with the conventional systems, it is found that the welded construction requires expensive non-destructive testing of the welds, especially to be sure that the welded structures can withstand the thermal stresses to which the vessel may be subjected during fabrication, testing or use. Because of the complex structure and testing problems mentioned, the conventional vessel is practically incapable of serial or mass production and is very expensive.